1. Field of the Invention
The present invention relates to a mixer, and more particularly, to a mixer reducing the time required for turning on a switching element in order to reduce 1/f noise.
2. Description of the Related Art
Direct transformation receivers have been studied as a receiver structure for realizing single chips. Such a direct transformation receiver can reduce external elements such as a filter or the like which burden the processing of a digital signal. Thus, the direct transformation receiver is most suitable for manufacturing single chips using a complementary metal-oxide semiconductor (CMOS) process by which digital circuits are easily realized. A direct transformation receiver may be classified into either a radio frequency (RF) direct transformation receiver transforming an RF signal into a base band signal or an intermediate frequency (IF) direct transformation receiver transforming an RF signal into a specific IF signal and then the specific IF signal into a base band.
A mixer used in such a direct, transformation receiver mixes an RF signal with a local oscillator (LO) signal to output a base band signal.
FIG. 1 is a circuit diagram of a conventional mixer using a CMOS Gilbert cell used in an RF direct transformation receiver. As shown in FIG. 1, the mixer includes an amplifier 20 and a mixing unit 10.
The amplifier 20 includes a pair of amplifier elements MA1 and MA2 and amplifies an input signal and provides the amplified signal to the mixing unit 10.
The mixing unit 10 includes first and second switching elements MS1 and MS2 switching an output current of the amplifier element MAl and third and fourth switching elements MS3 and MS4 switching an output current of the amplifier element MA2. A local oscillator signal LO+ is input from a local oscillator to gates of the first and fourth switching elements MS1 and MS4, and a local oscillator signal LO− is input from the local oscillator to a common gate of the second and third switching elements MS2 and MS3. Here, the local oscillator signal LO+ input to first and fourth switching elements MS1 and MS4 is 180° out of phase with the local oscillator signal LO− input to the second and third switching elements MS2 and MS3. Drains of the second and fourth switching elements MS2 and MS4 are connected to each other, and drains of the third and first switching elements MS3 and MS1 are connected to each other.
The mixing unit 10 mixes a signal amplified by an amplifier element with a local oscillator signal to output a signal corresponding to a difference between frequencies of the signals. The amplifier elements MA1 and MA2 and the first, second, third, and fourth switching elements MS1, MS2, MS3, and MS4 constituting the mixer are realized as n-channel metal-oxide semiconductor field effect transistor (MOSFET) elements.
However, if the mixer uses such a MOSFET and a current is provided to the first, second, third, and fourth switching elements MS1, MS2, MS3, and MS4, the first, second, third, and fourth switching elements MS1, MS2, MS3, and MS4 require a predetermined turn-on time to be turned on and reach a target voltage. Such a turn-on time increases 1/f noise additionally offsetting a direct current (DC) and deteriorating a noise figure. In particular, the 1/f noise increases with changes from a high frequency to a low frequency. Thus, the first, second, third, and fourth switching elements MS1, MS2, MS3, and MS4 cause such problems in an RF direct transformation receiver outputting a low frequency base band signal.
io,n indicating the 1/f noise output from the mixer can be expressed as in Equation 1:
                              i                      o            ,            n                          =                                            2              T                        ⨯            2                    ⁢                      I            ⨯            Δ                    ⁢                                          ⁢          t                                    (        1        )            According to Equation 1, a time T may be increased, a supplied current I may be decreased, or a turn-on time Δt may be decreased in order to decrease the 1/f noise. However, a frequency must be lowered to increase the time T. In general, since a receiver and a transmitter use a frequency in a specific band, it is impossible to lower the frequency. If the current I is reduced, the performance of the mixer may be deteriorated. Thus, decreasing the turn-on time Δt may be desirable. Therefore, a method of decreasing the turn-on time Δt of the mixer is required to reduce the 1/f noise in the RF direction transformation receiver.